A variety of three-dimensional structures having protrusions formed on both sides of a resin sheet have been suggested (JP-B-S62-15330, JP-B-H5-12139, etc.). However, the suggested structures have a high rigidity; in particular those with a thickness (height of three-dimensional structure) exceeding 6 mm, cannot be wound into large rolls, and are unsuitable for applications in the form of sheet materials that are effective when a large surface area is needed. In the structure with both side protrusions described in JP-B-S62-15330, because the tip portions of the protrusions are flat and portions thereof are not deformed, the thickness is large, no contribution is made to increasing the compressive strength, and an unnecessarily large amount of valuable resources are used. Moreover, when the structure is thermally melted to joint to other sheets, a large amount of thermal energy is needed for melting. The resultant drawback is that thin portions on the protruding side surfaces are deformed by this extra heat. Yet another undesirable feature of the structure with flat tip portions, which is described in the aforementioned reference, is that because the compressive strength increases with the number of protrusions, the number of protrusions cannot be increased. Moreover, because the tip portions of protrusions are flat, the flexibility of the structure is lost accordingly.
Manufacturing a three-dimensional structure with protrusions on both sides from a resin sheet with both side embossed rolls, as suggested in JP-B-H5-12139, requires a pulling-out angle. Therefore, the molding process places limitations on the pitch and height of the projections obtained by embossing and a structure with high needle-like protrusions cannot be obtained with a small pitch. Furthermore, needle-like protrusions can be also manufactured by electric discharge processing (JP-A-2000-17091), but the absolute height of the peaks cannot be increased. Furthermore, because only one surface can be processed in one cycle of processing, a structure with protrusions on both sides is difficult to manufacture. A resin three-dimensional structure with protrusions on both sides can be also manufactured by injection molding with dies, but because the moldings have to be pulled out from the dies, deep valleys cannot be manufactured. Moreover, the resin sometimes cannot be processed to obtain thin sections of the tips, continuous sheets cannot be molded, die manufacturing cost is high due to a complex shape, and the production cost becomes high.
Furthermore, foamed sheets from polyethylene resin are used as flexible bulky structures, but the sheets foamed to high bulkiness have a low compressive strength, poor water permeability, low resistance to surface wear, and poor mechanical dimensional stability. Therefore, there is a need for a flexible and bulky material that is air and liquid permeable and has a high compressive strength and good mechanical dimensional stability. Furthermore, even with foamed polyethylene sheets, a product with a thickness of 5 mm or more is technologically difficult to produce, therefore sheets with such a thickness are obtained by joining together thick foamed sheets. However, in this case, the amount of work is doubled and because cooling the intermediate sections in the joining process requires time and the line speed is low, the production efficiency is poor.